It is not easy to measure liquid levels, especially where the liquids are hazardous or flammable.
It is well known in the art to provide a float within a container, the float caused to rise and fall by the level of liquid in the container. The float is linked to a rotating first magnet that is within the container, or is at least on the liquid side (inside) of a divider that is joined to an opening in the container. The first magnet is magnetized so that a magnetic moment has a nonzero component at a right angle to the axis of rotation, and preferably its moment is entirely at a right angle to that axis. A second rotating magnet is on the outside of the divider, and is nearly coaxial with and magnetically coupled with the first magnet, likewise having a moment (or a component of the moment) perpendicular to the axis. The second magnet may actuate a pointer providing a human-readable indication of the liquid level. The second magnet is physically nearby to a magnetic flux sensor such as a Hall-effect sensor. The flux sensed in the sensor is indicative of the liquid level. The sensed flux signal is converted from analog to digital and is passed on to other equipment. Mechanisms suitable for use in such apparatus are described, for example, in U.S. Pat. Nos. 4,987,400, 6,041,650, and 6,089,086, incorporated herein by reference.
Unfortunately, this approach offers many drawbacks. One drawback is that with the conventional and commonly used magnetization and magnet shape, the sensed flux deviates substantially from linearity with respect to the actual liquid level. While such nonlinearity can be corrected in software (after the A/D conversion, for example), this results in variations in resolution across the range of measured physical values such as liquid level, and adds to computational cost.
Yet another drawback is that with such magnetization and magnet shape, it is impossible to disambiguate certain distinct liquid levels based solely on the sensed flux at a particular time; for disambiguation it is necessary to maintain state information such as historical information about recent sensed values. Such disambiguation requires frequent data collection and depends upon assumptions regarding how quickly the liquid level might change. The disambiguation problem may be avoided by limiting the permitted angular rotation of the magnets, for example by choosing the details of the float linkage, such as gear ratios. This has the drawback of limiting either the resolution of the sensing system or the dynamic range of the sensing system, or requiring a more expensive analog-to-digital convertor.
It is thus desirable to provide a system for measurement of liquid levels or other physical phenomena, which employs a float or other follower linked to a first magnet, and a second magnet linked to the first magnet, where the electrical output is nearly linear with the physical phenomenon being measured, and wherein the dynamic range is maximized and resolution uncompromised, all without expensive post-processing of data and without expensive high-resolution A/D convertors.
A liquid level sensor is provided for use with a container. The sensor protrudes through an opening in the container. The sensor includes a float linked by means of a linkage to a first magnet axially rotatable on a first axis. The first magnet has a magnetic moment with a nonzero component at a right angle to the axis. A divider separates the first magnet from a second magnet having a magnetization and axially rotatable on a second axis. The second magnet has a nonzero magnetic moment at a right angle to the second axis. The first and second magnets are juxtaposed in magnetic linkage so that the second magnet is urged to follow the first magnet in rotation. The divider plugs the opening in the container. A magnetic field sensor is positioned to sense axial magnetic field strength at a location offset from the second axis. Importantly, the magnetization of the second magnet gives rise to a sensed magnetic field at the sensor that is nonsinusoidal with respect to an angle of rotation of the second magnet on the second axis, or gives rise to north and south poles separated by more than a half-circle or axial rotation of the second magnet.